Can I tie a washing machine into a bathtub drain line?

Is there any reason I would not want to tie a new washing machine into a bathtub drain as long as I give the washing machine its own P trap? One plumber I spoke with was concerned the force of the washing machine "would suck the water right out of the trap" of the bathtub. Does that sound right? The washing machine is a high efficiency model and uses very little water. Also, can I use an AAV to vent the washing machine?

EACH trap needs its own individual vent. A tub drain might be as small as 1.5". A washing machine drain needs to be at least 2". So, probably not, but it would depend on how things are layed out. If each trap is properly vented, then, no, neither should siphon the other. Some places do not allow an AAV, so even if it might work, it would not be to code. It is always better to do an atmospheric vent if possible. Keep in mind that an AAV can only let air in, and depending on where it is, sometimes air needs to come out as well.

I believe the question of AAV on a washer was addressed recently, and if I remember correctly, the can not be used for the reason Jim noted on an AAV only letting air in. Also as noted, washers now require a 2" drain.

Is there any reason I would not want to tie a new washing machine into a bathtub drain as long as I give the washing machine its own P trap?

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It's more about the venting.
Your plumber has mentioned that water from the washer discharge will suck the tub trap dry.
If you were to go downline of the tub line, after it has been vented, and then branch off the 2" waste line, with an above floor trap and vent for the washer, that would work. The two should work independent of each other.

A washer discharges with a pump. Some locations require a 3" line after the 2" trap and trap arm.

It's more about the venting.
Your plumber has mentioned that water from the washer discharge will suck the tub trap dry.
If you were to go downline of the tub line, after it has been vented, and then branch off the 2" waste line, with an above floor trap and vent for the washer, that would work. The two should work independent of each other.

A washer discharges with a pump. Some locations require a 3" line after the 2" trap and trap arm.

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Terry is there not also a certain fall that needs to be realized? I think its something like 36".